The present invention relates to an in-situ method for determining the pore size distribution, capillary pressure curve and permeability of a formation surrounding a well. These are all important considerations in deciding whether hydrocarbon-bearing formations are commercial. A formation may contain a large amount of hydrocarbon but if the pores are too small and the permeability is too low it may not be possible to produce the hydrocarbons commercially. Thus, the determination of the pore size distribution, capillary pressure curve, and permeability of the formation is an important consideration in determining whether a well which has penetrated a hydrocarbon-bearing formation should be completed. The expense of completing a well is, of course, considerable since the well must first be cased and production tubing and well-head equipment installed.
At present there are only two reliable methods for determining the pore size distribution, capillary pressure curve and permeability of a formation. The method used most often is to take cores from the formation during the drilling of the well and then to analyze the cores in a laboratory. Coring enables high precision laboratory measurements of capillary pressure and permeability. Coring, however, is very expensive and involves some risk. While coring a well, one does not have as much control over the well as when drilling with conventional rock bits and the possibility of sticking the drill string in the well is greatly increased during coring operations. Further, during coring operations, the penetration rates are greatly reduced, thus increasing the time required to drill the well to a target depth. In addition, measurements on core plugs may not be a good representation of the formation as a whole, since core plugs are small, discrete samples.
The only reliable logging method for determining pore size distribution, capillary pressure curves and permeability is the use of the nuclear magnetism logging (NML) tool. In this method the T.sub.1 decay curve of protons magnetically polarized in the formation is recorded and then mathematically inverted to obtain a pore size distribution and an approximate permeability (J. D. Loren and J. D. Robinson, "Relations Between Pore Size, Fluid and Matrix Properties, and NML Measurements", Society of Petroleum Engineers Journal, September 1970, pages 268-278). The use of nuclear magnetism logging is commercially practiced and readily available. While the technique is available, it does have several disadvantages. Among the disadvantages are the small depth of investigation of the nuclear magnetism log (about 6 inches) and the requirement that the borehole fluid be treated with magnetized particles to eliminate the borehole mud response from the log. Normally, magnetite particles in suspension are added to the drilling mud to suppress the borehole response. This involves considerable expense and effort, because it is necessary to circulate the magnetite in the well to assure adequate mixing, and the circulated mud must be checked to verify that the nuclear magnetic response has been eliminated.
Another disadvantage of the nuclear magnetism log is that the signal is extremely weak, often requiring several measurements at a fixed location for signal averaging. Thus, a good NMR decay curve of the quality required for capillary pressure and permeability determination is usually not obtained while continuously logging. Another disadvantage of the NML is that the nuclear magnetism decay times are so short in some formations, for example in tight gas sands, that the signals cannot be measured by existing NML tools (J. A. Brown, L. F. Brown, J. A. Jackson, J. V. Milewski, B. J. Travis, "NMR Logging Tool Developments: Laboratory studies of Tight Gas Sands and Artificial Porous Material", SPE/DOE 10813, pages 203-208). This is a severe disadvantage because it is precisely the marginal, low permeability formations where accurate measurement of the permeability is required most. Still another disadvantage of the NML is that the permeability determined from nuclear magnetism is a three-dimensional average, because the polarized protons on the water molecules diffuse randomly until they relax at the pore walls. In many formations there is an order of magnitude difference between vertical and horizontal permeability. The NML results will yield a three-dimensional averaged permeability rather than a separate vertical and horizontal permeability. Yet another difficulty with nuclear magnetism logging is that in oil-bearing formations, both the protons in water and in oil contribute to the T.sub.1 decay curve. Since the water phase and the oil phase have different decay rates, an unambiguous determination of pore size often cannot be made. In addition, the T.sub.1 decay times depend on the character of the pore surface, such as hydrocarbon wetting, bound hydration ions and paramagnetic centers. As is well known in the art, these can all produce significant effects on the measured T.sub.1 relaxation times (J. A. Glasel, "NMR Relaxation in Heterogeneous Systems", Nature 227, 704-705 (1970); R. J. S. Brown and I. Fatt, "Measurements of Fractional Wettability of Oilfield Rocks by the Nuclear Magnetic Relaxation Method", Pet. Trans. AIME 207, 262-264 (1956).)
A final problem in relating the NMR relaxation times to formation permeability is that the pores probed by NMR need not be hydraulically connected. Therefore an impermeable medium containing disconnected vugs could yield the same T.sub.1 decay curve as a permeable rock containing connected pores.